Magnetic resonance tomography (MRT) is an imaging method which enables the high-resolution generation of sectional images of living organisms, such as humans. The patient is placed in a homogeneous magnetic field B0. Using gradient coils the outer magnetic field in the FOV (field of view) is modified in such a way that firstly, a body layer is selected and secondly, the generated magnetic resonance (MR) signals are spatially encoded.
During the subsequent reconstruction of the MR signals, by way of example by Fourier transformation, an image of the selected layer is produced which is used for medical diagnosis. The MR signals are generated and detected using a high-frequency system, comprising a transmitting antenna, which radiates high-frequency (HF) excitation pulses into the patient, and a receiving antenna, which detects the emitted HF resonance signals and forwards them for image reconstruction. The contrast of the MR images can be varied in many ways depending on the diagnostic task by the selection of a suitable pulse sequence, such as a spin echo sequence or a gradient echo sequence, and the sequence parameters pertaining thereto. The MRT images body structures and therefore provides a structural imaging method.
Movements during an MR scan, such as respiratory movements of a patient who is to be examined by way of MR, can lead in magnetic resonance imaging, primarily in the case of an examination of the organs of the thorax and the abdomen, in particular of examination areas affected by the respiratory movement of the patient, to artifacts, such as what is known ghosting or blurring and/or to a loss of intensity in the generated images and to registration errors between generated images. These artifacts can make a diagnosis on the basis of these images, for example by a physician, difficult and can lead to, for example, lesions being overlooked.
Numerous methods exist in the prior art for reducing artifacts as a consequence of, by way of example, a respiratory movement. One of these methods is the activation of a trigger signal to detect magnetic resonance image data as a function of a respiratory movement or, generally, what is known as breathing gating. Breathing gating is a method in which the breathing of the patient is detected during the MR measurement and is associated with the acquired measurement data. With breathing gating the measurement data is only used for reconstruction if the detected respiratory movement meets certain predefinable criteria.
The breathing of the patient can be detected in this connection using external sensors, for example a pneumatic cushion, or using MR signals, what are known as navigators. A navigator is usually a short sequence which acquires MR signals for example from the diaphragm or another signal source in the examination object, whose movement is correlated with the breathing of the patient. The respiratory movement can be traced by way of the position of the diaphragm or the other signal source.
In the case of breathing gating with navigators, the navigator sequence is interleaved by way of example with the imaging sequence and a diaphragm position measured using a navigator is then associated with the imaging data acquired immediately thereafter or therebefore.
A distinction is made between retrospective and prospective breathing gating.
With retrospective breathing, gating the respiratory movement is detected and recorded during the MR measurement, but is not evaluated. Instead, the k-space that is to be detected is measured several times. Only some of the measured data is used for reconstruction, and preferably that in which the breathing signal is in a specific window around an excellent breathing position. If a specific k-space data point that is required for image reconstruction was measured several times within the excellent window, the data can be averaged. If, on the other hand, a data point was always measured outside of the window, the data point which has the smallest deviation from the excellent position can be used for reconstruction.
With prospective breathing gating the physiological breathing signal measured with the aid of a breathing sensor (for example the diaphragm position measured using a navigator sequence) is evaluated during the measurement and the MR measurement controlled on the basis of the detected physiological signal. In the simplest embodiment, what is known as the acceptance/rejection algorithm (ARA), the measurement of an imaging data packet (and optionally the associated navigator sequence) is repeated until the physiological signal falls within a previously defined acceptance window.
A further possibility for artifact reduction resides in a movement-compensated reconstruction. In this case the image data is segmented into states of different breathing stages after detection of respiration. After a reconstruction of the images for the corresponding breathing stages a movement model is estimated by way of a registration, with the aid of which model a movement-free image volume is in turn reconstructed.
The various methods also have different signal-to-noise ratios. A priori it is not clear which method delivers the best results.
DE 10 2012 213 551 A1 relates to a method for movement-averaged attenuation correction of PET data. For this purpose MRT data is sorted by various gating methods according to its movement phase, in particular breathing phase, with all data records emerging from the sorting process continuing to be considered, however, and being averaged later.
DE 10 2012 206 555 A1 relates to a method for the acquisition of a magnetic resonance data record of a breathing examination object, with the breathing phase being estimated by a state machine for prospective gating.
The article “Thoracic respiratory motion estimation from MRI using a statistical model and a 2-D image navigator” by A. P. King et al., Medical Image Analysis 16 (2012), 252-264, relates to the magnetic resonance-based movement correction of PET data.